When a document is printed using multiple print engines, especially when printed duplex on a continuous feed machine, pages or sides printed on different print engines may have different color appearance, unless the color rendering is altered to take into account printer-to-printer differences. One known system provides pages that are RIPped to an engine-neutral space and then once the engine is known, they are color transformed according to the engine. To do this well requires storing continuous tone data, and also applying the color transform to raster data.
For example, some systems render pages to (compressed) bitmaps, while converting the colors to an engine-neutral color space, and then color-convert the output bitmaps to account for inter-engine differences. While this can be advantageous for a particular application, if it is difficult to determine ahead of time which engine will be used for which page, there are disadvantages to this approach as well. One disadvantage is that every pixel must be converted, even though there may be large areas of a single color. Once the page has been converted to a bitmap, it is more difficult to detect regions of constant color. Another disadvantage is that the page must be rasterized in continuous tone, rather than one-bit deep. It is common for high speed systems to use a binary image path, in which colors are converted to binary through halftoning early in the process, thereby reducing the amount of data to be transferred in later stages. Once an image is converted to binary it is difficult (but not impossible) to change its tone scale, and even more difficult to apply a color space transformation that includes inter-separation dependencies.
When a job is printed by multiple print engines, there is the opportunity for pages to exhibit mis-match due to the difference between engines. This difference may be compensated through calibration or profiling techniques, to make the pages match (or at least more nearly than without compensation). This disclosure and the embodiments described herein, describe an efficient way of applying color compensation in a page parallel RIP system. Such a system includes the arrangement of multiple printers used to print a document, where the output stacks are arranged in such a way that they may readily be combined. Further, arrangements are provided for continuous feed systems in which two engines are connected so that one engine prints front while the other prints back sides of a document. It is with reference to this last case that the present disclosure is described, however other embodiments are possible. In particular, where two engines are referenced, or “first” and “second” are mentioned, more engines could be used.
The subject disclosure relates to printing systems, their architecture and processing steps and, more particularly, to producing electronic images in a manner to drive digital printers more efficiently and with image appearance consistency.